Page 61 - Low Temperature Energy Systems with Applications of Renewable Energy

50 Low-Temperature Energy Systems with Applications of Renewable Energy

When choosing a heat pump of the “air-air” type, one must pay attention to mini-
mizing electric power supply to ventilators and to overcoming aerodynamic air duct
resistance. It is recommended to keep the total electrical cost under 1 W/m/h of air.
Air velocity in the heat exchangers should not exceed 2.5 m/s. Velocity control is
made easier by installing two compressors.
The heat pump assembly should be located at an area adjacent to a multi-story
building or in a cellar, taking measures to lower vibration and noise. The air duct
must be connected with flexible heat pump elements. Condensate must go into the
sewer system. The outlet of the heated air into the rooms should be done near the outer
walls in main rooms or through holes in the floor or making the floor from thermo-
porous material which can pass warm air through itself. All air ducts must be
effectively insulated. When the air duct goes through a room being not heated, the
insulation must be not less than 50 mm thick, and through a room being heated, about
2e3 mm thick. Absorbing air ducts for the collection of the exhaust air must be placed
in kitchens, lavatories and bathrooms. They must not be near the doors of rooms that
are not heated. Supplementary heaters (electric heaters generally) are placed in each
room that is heated. They cannot be located under exhaust air ducts.

2.2.1.2 “Ventilation air-water” heat pump
Air ducts cannot always be installed in existing and new houses which is why water is
often used to distribute heat energy in rooms. In buildings having mechanical ventila-
tion, air is taken in from certain rooms such as kitchens and bathrooms and directed to
the heat pump evaporator. Water that is heated in the heat pump condenser goes to the
heaters and its temperature should not be higher than 50 C in order to maintain a high

COP. Low heat-carrier temperature requires increasing the heat transfer surface area,
which is why embedding the heat exchangers inside the floor is the best solution in
new buildings. In old ones it is necessary to install additional thermal insulation on
the walls and to change windows so that the existing radiators can be saved without
increasing their heat exchange surface area.
Heat pump power is chosen depending on ventilation air consumption calculated
for ventilation of 70e80% of room volume per hour, and during very cold weather,
conventional electric-resistance heaters are turned on in rooms or a centralized boiler
is activated. In common multi-story buildings the use of ventilation air is possible if
there are ventilation channels, however, in many older buildings such channels are
not available.
It is important to note that the use of ventilation air is undesirable in cases where the
building is not completely inhabited because the temperature of the ventilation air will
be insufficient and will drop below the recommended minimum values, typically

15 C. In case of cold weather, a supplementary alternative energy source, usually
direct electric resistance heating, is used in the same way as in private dwellings. A
thermostat must be installed for the electric heaters. Maximum usage of the heat
pump is the priority of temperature control in the room. Experiments have shown
that the use of a ventilation-air heat pump can decrease electric energy consumption
by 40% compared with the direct heating [6].